This invention relates to a method of converting a liquid flame retardant into a more easily handled powder. In particular, it relates to heating a mixture of a liquid tetrabromophthalate flame retardant and polyvinyl chloride (PVC) to produce a powdered flame retardant composition.
A flame retardant is often added to PVC and other resins to reduce the flammability of the resin so that the resin will pass industry flammability tests. A widely used flame retardant is di-2-ethylhexyl tetrabromophthalate (DEHTBP). While this flame retardant is very effective, it is a viscous liquid and cannot be easily handled without heating it to reduce its viscosity. This is an additional step in the manufacturing process and requires additional energy.
I have discovered that DEHTBP and similar liquid flame retardants can be converted from liquids into a free-flowing powders by mixing them with PVC and heating the mixture. The resulting flame retardant composition is easy to handle and can be added to a resin without additional heating. When the flame retardant composition is added to a polymer, it is as effective as the liquid flame retardant itself, and the properties of the resin are not adversely affected.
This invention is applicable to liquid flame retardants having the general formula 
where R1 and R2 are independently selected from aliphatic from C2 to C20. Examples of R1 and R2 groups include ethylhexyl, octyl, nonyl, septyl, hexyl, decyl, and methylseptyl. Preferably, R1 is the same as R2 and they are branched or straight chain alkyl from C6 to C10as those flame retardants are the best plasticizers. DEHTBP is most preferred because it is a commercial product.
To prepare the dry, solid, flame retardant, powdered composition of this invention, the liquid flame retardant is mixed with dry PVC powder. The PVC is preferably a homopolymer as homopolymers have better absorption, but it can also be a copolymer of vinyl chloride and up to 25 wt % of another compatible monomer, such as vinyl acetate. The PVC particle size can be from about 50 to about 300 microns; a preferred particle size is about 150 to about 200 microns. About 20 to about 150 phr (parts per hundred parts PVC by weight), and preferably about 70 to about 130 phr, of the flame retardant is mixed with the PVC. Less flame retardant is not very useful and it is hard to absorb more flame retardant onto the PVC.
Various optional components can be included in the mixture, such as about 0.1 to about 10 phr of a thermal stabilizer such as a tin compound, a cadmium compound or a barium/cadmium compound; about 5 to about 100 phr of a plasticizer, such as dioctyl phthalate (DOP); or about 1 to about 20 phr of a synergist such as antimony oxide.
The mixture is heated at between about 50 and about 100xc2x0 C. At lower temperatures the liquid flame retardant does not absorb well onto the PVC and at higher temperatures the PVC may fuse (melt). A preferred temperature range is about 80 to about 100xc2x0 C. The friction from the mixing usually supplies enough heat, though additional heating or cooling may be necessary. The mixing is preferably intense, in a high speed mixer, such as a Henschel mixer. When the torque of the mixer falls off, the mixing is usually finished. The resulting flame retardant composition has about the same particle size as the PVC powder used to make it.
The flame retardant composition can be added to a variety of polymers, including PVC, polyethylene, polypropylene, polystyrene, polystyrene foam, polyethylene vinyl acetate, polyethylene ethyl acetate, and ethylene propylene butadiene rubber; preferably, it is added to PVC or to polypropylene. The amount of flame retardant composition needed can be experimentally determined by adding increasing amounts to samples of the polymer until a sample passes whatever flammability test is being used. This might require only about 5 wt % in polystyrene foam, but as much as 100 wt % in polypropylene. In PVC, the amount of flame retardant composition is preferably about 20 to about 160 wt % as that is usually sufficient to pass a flammability test such as ASTM 02863.